Conventionally, as a machining center which moves a large-size work and a tool relative to each other in a three-dimensional manner to thereby carry out working operations such as a cutting operation and a drilling operation, for example, there is used a portal machining center which not only moves straight a table with a work mounted thereon in a reciprocating manner but also controls a spindle including a tool in the X axis, Y axis and Z axis directions.
In the portal machining center, a saddle is mounted on a cross rail supported by two columns, a spindle head is mounted on an end portion of a ram movable in the Z axis direction relative to the saddle, and further a motor built-in type spindle is swingably mounted on the two support arms of the spindle head through a bracket (see, for example, the patent reference 1).
Also, in a spindle device included in an ordinary machine tool, a rotation joint for supplying liquid such as coolant to a tool is disposed within a spindle, and the fluid is supplied through a piping to the rotation joint from a liquid supply apparatus provided externally of the spindle device (see, for example, the patent reference 2).    Patent Reference 1: International Patent Publication No. WO96/41695 (FIG. 4)    Patent Reference 2: Japanese Patent Unexamined Publication JP-A-2001-287102 (FIG. 4)